# Can't get http\_parameters to work

**URL:** <https://swi-prolog.discourse.group/t/cant-get-http-parameters-to-work/2043>\
**Category:** Predicate\
**Created:** [March 20, 2020, 7:05am UTC](https://swi-prolog.discourse.group/t/cant-get-http-parameters-to-work/2043 "2020-03-20T07:05:22Z")\
**Posts on this page:** 5\
**Page:** 1

<div class="post-metadata">

**Author:** ![RdR](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/rdr/32/379_2.png) [@RdR](https://swi-prolog.discourse.group/u/RdR)\
**Post date:** [March 20, 2020, 7:05am UTC](https://swi-prolog.discourse.group/t/cant-get-http-parameters-to-work/2043/1 "2020-03-20T07:05:23Z")

</div>

I cannot seem to get http\_parameters to work and I’m not sure why (swipl 8.0.3 64bit). I’ve created a tiny server example below, which I test from the linux commandline with httpie (like curl, but nicer interface). When I debug the Request data, it looks like the JSON data has disappeared, but I’m not how that happens.

Here is the code for the tiny server (and the interchange with the server below that):

```prolog
% http libraries
:- use_module(library(settings)).
:- use_module(library(http/thread_httpd)).
:- use_module(library(http/http_dispatch)).
:- use_module(library(http/http_path)).
:- use_module(library(http/http_parameters)).
:- use_module(library(http/json)).
:- use_module(library(http/json_convert)).
:- use_module(library(http/http_json)).
:- use_module(library(debug)).

% handler declarations
:- http_handler(root(.), frontpage,[prefix]).

% server
server:-
    open('log.txt',append, ErrStream,[]),
    debug(server>ErrStream),
    http_server(http_dispatch,[port(8080)]).

stopserver:- http_stop_server(8080,[]).

frontpage(Request):-
    http_parameters(Request,[
        email(Email, [optional(true)]),
        password(_Passwd, [optional(true)])
    ]),
    debug(server,'~w',[Request]),
    (var(Email)->Email='empty email field';true),
    reply_json( json([user=Email]) ),!.

```

With httpie, on the commandline, I fire a POST request (also tried GET and PUT) as follows:

```prolog
http -v POST :8080 email=john@mail.com password=fubar

```

This causes httpie to send the following to the server:

```prolog
POST / HTTP/1.1
Accept: application/json, */*
Accept-Encoding: gzip, deflate
Connection: keep-alive
Content-Length: 47
Content-Type: application/json
Host: localhost:8080
User-Agent: HTTPie/0.9.8

{
    "email": "john@mail.com",
    "password": "fubar"
}

```

And the server responds as follows:

```prolog

HTTP/1.1 200 OK
Connection: keep-alive
Content-Length: 28
Content-Type: application/json; charset=UTF-8
Date: Fri, 20 Mar 2020 06:47:03 GMT

{
    "user": "empty email field"
}

```

When I inspect the log file, the contents of the Request are as follows:

```prolog
% [Thread httpd@8080_13] [protocol(http),peer(ip(127,0,0,1)),pool(client(httpd@8080,user:http_dispatch,<stream>(0x7f07b0
023070),<stream>(0x7f07b0011af0))),input(<stream>(0x7f07b0023070)),method(post),request_uri(/),path(/),http_version(1-1)
,host(localhost),port(8080),user_agent(HTTPie/0.9.8),accept_encoding(gzip, deflate),accept([media(application/json,[],1.
0,[]),media(_2460/_2462,[],1.0,[])]),connection(keep-alive),content_type(application/json),content_length(47)]

```

---

<div class="post-metadata">

**Author:** ![CapelliC](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/capellic/32/17_2.png) [@CapelliC](https://swi-prolog.discourse.group/u/CapelliC)\
**Post date:** [March 20, 2020, 8:35am UTC](https://swi-prolog.discourse.group/t/cant-get-http-parameters-to-work/2043/2 "2020-03-20T08:35:19Z")

</div>

Copied your code in a local folder, and started `?- server.` .

Now I can access it from the browser at the url `http://localhost:8080/?email=john@mail.com` to get the JSON reply `{"user":"john@mail.com"}` .

So, GET parameters are working as expected. To show that POST parameters works as well, have run this PHP snippet:

```prolog
<?php

// Your POST data
$data = http_build_query(array(
    'email' => 'john@mail.com',
    'password' => 'fubar'
));

// Create HTTP stream context
$context = stream_context_create(array(
    'http' => array(
        'method' => 'POST',
        'header' => 'Content-Type: application/x-www-form-urlencoded',
        'content' => $data
    )
));

// Make POST request
$response = file_get_contents('http://localhost:8080/', false, $context);
print_r($response);

```

and got the expected `{"user":"john@mail.com"}` .

Seems that httpie doesn’t encode the FORM data as expected…

The log.txt (but you forgot the enabling directive `:- use_module(library(http/http_log)).`) relevant entries entries are

```prolog
% [Thread httpd@8080_5] [protocol(http),peer(ip(127,0,0,1)),pool(client(httpd@8080,user:http_dispatch,<stream>(0x7f77040108d0),<stream>(0x7f7704010a10))),input(<stream>(0x7f77040108d0)),method(post),request_uri(/?email=cc@cc.cc),path(/),search([email=cc@cc.cc]),http_version(1-0),host(localhost),port(8080),connection(close),content_length(36),content_type(application/x-www-form-urlencoded)]
% [Thread httpd@8080_3] [protocol(http),peer(ip(127,0,0,1)),pool(client(httpd@8080,user:http_dispatch,<stream>(0x7f770c0127c0),<stream>(0x7f770c0128d0))),input(<stream>(0x7f770c0127c0)),method(post),request_uri(/),path(/),http_version(1-0),host(localhost),port(8080),connection(close),content_length(36),content_type(application/x-www-form-urlencoded)]

```

Guess httpie should be instructed to pass `content_type(application/x-www-form-urlencoded)` …

---

<div class="post-metadata">

**Author:** ![jan](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/jan/32/4_2.png) [@jan](https://swi-prolog.discourse.group/u/jan)\
**Post date:** [March 20, 2020, 8:42am UTC](https://swi-prolog.discourse.group/t/cant-get-http-parameters-to-work/2043/3 "2020-03-20T08:42:31Z")

</div>

Yip. On the other hand it might get time to make http\_parameters/2 also understand JSON POST content instead of only `x-www-form-urlencoded`. This is slightly different as both GET and worm-encoded POST provide the parameters as untyped text and this http\_parameters/2,3 does the necessary translation to e.g. a number. Using JSON we already have dynamically typed numbers. Still, I guess it would fit nicely and may simplify simple JSON POST handlers.

---

<div class="post-metadata">

**Author:** ![RdR](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/rdr/32/379_2.png) [@RdR](https://swi-prolog.discourse.group/u/RdR)\
**Post date:** [March 20, 2020, 6:57pm UTC](https://swi-prolog.discourse.group/t/cant-get-http-parameters-to-work/2043/4 "2020-03-20T18:57:22Z")

</div>

Thanks CapelliC – changing the content\_type solved the issue.

FYI, the way to get httpie to use the correct content\_type is:

```prolog
http --form POST :8080/login email='john@mail.com'

```

(btw, I used debug instead of http\_log – that’s why you didn’t see me use the http\_log library).

Thanks again!  
PS. Jan, I agree with your suggestion that it would be nice to have http\_parameters accept json data more directly.

---

<div class="post-metadata">

**Author:** ![CapelliC](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/capellic/32/17_2.png) [@CapelliC](https://swi-prolog.discourse.group/u/CapelliC)\
**Post date:** [March 21, 2020, 8:25am UTC](https://swi-prolog.discourse.group/t/cant-get-http-parameters-to-work/2043/5 "2020-03-21T08:25:55Z")

</div>

> [@RdR](#):
>
> Thanks again

You’re welcome!

> [@RdR](#):
>
> I used debug instead of http\_log

Oops, I just noticed log.txt was empty after a couple requests, and missed the debug call(s)… guess the inclusion of http\_log didn’t changed anything wrt logging, but probably I restarted the server and got log.txt flushed… not sure…